Method for liquefying coal

ABSTRACT

A TWO-STAGE SOLVENT EXTRACTION METHOD FOR CONVERTING SOLID COAL INTO LIQUID COAL EXTRACT. THE FIRST STAGE USES THE CONVENTIONAL SOLVENTS, SUCH AS POLYAROMATIC HYDROCARBONS ALKYLNAPHTHALENES, ANTHRACENE OIL, OR PARTIALLY HYDROGENATED AROMATICS AS FOR INSTANCE, TETRALIN. THE LIQUEFIED COAL STRIPPED OF THE SOLVENT IS THEN EXTRACTED WITH A KETONE TO PRODUCE A HYDROGEN-RICH EXTRACT FRACTION AND A HYDROGEN   LEAN EXTRACT PHASE. THE HYDROGEN-RICH FRACTION IS RECOVERED AND USED AS A SOURCE FOR VALUABLE CHEMICALS AND LIQUID FUELS. THE HYDROGEN-LEAN FRACTION MAY BE USED FOR PLANT FUEL AND/OR AS A SOURCE FOR ADDITIONAL HYDROGEN THROUGH CONVERSION BY THE &#34;WATER-GAS&#34; REACTION.

Aug. 10, 1971 G, R, SUNAGEL ETAL 3,598,717

A METHOD FOR LIQUEFYING COAL Filed June 25. 1968 By'm., W. @my

A TTORNEYS United States Patent 3,598,717 METHOD FOR LIQUEFYING COAL George R. Sunagei, Lincolnwood, Richard S. Corey, Rolling Meadows, Frederick J. Riedl, Arlington Heights,

ABSTRACT OF THE DISCLOSURE A two-stage solvent extraction method for converting solid coal into liquid coal extract. The tirst stage uses the conventional solvents, such as polyaromatic hydrocarbons, alkylnaphthalenes, anthracene oil, or partially hydrogenated aromatics as for instance, Tetralin. The liquefied coal stripped of the solvent is then extracted with a ketone to produce a hydrogen-rich extract fraction and a hydrogenlean extract phase. The hydrogen-rich fraction is recovered and used as a source for valuable chemicals and liquid fuels. The hydrogen-lean fraction may be used for plant fuel and/or as a source for additional hydrogen through conversion by the water-gas reaction.

BACKGROUND OF THE INVENTION This invention relates to a solvent extraction method. It also relates to a method for liquefying coal using selective solvents. It particularly relates to a two-stage solvent extraction method for converting solid coal into liquid coal extract.

It has long been known that hydrocarbon gases, liquids, pitch, and the chemicals derived from or allied to these hydrocarbons, may be obtained in some form from coal which is mined from the earth. Usually, the prior art has employed destructive distillation or other gasification processes for the conversion of coal into these more valuable and useful products. Recently, the prior art has developed high pressure hydrogenation techniques to convert coal into valuable liquid products. Still more recently, methods involving solvent extraction techniques have been otiered by the prior art for obtaining useful fuels and chemicals from coal wherein, broadly, the crushed coal is contacted with a selective solvent which acts at least in part as a hydrogen-donor for supplying suicient hydrogen to the hydrogen decient coal to aid in converting solid particulate coal to a liquid phase.

Following the solvent extraction step, the prior art schemes have generally utilized various product recovery procedures, such as hydrogenation of the liquid coal extract, for increasing its value and utility, together with retorting or coking of the residual materials obtained from the solvent extraction step to still further convert these coal derived products into more useful and valuable products.

One of the main dilculties in the liquefaction of coal using the solvent extraction process is in the separation step following extraction, i.e. the separation of the undissolved coal and inorganic material from the liquid. The prior art has attempted to make this separation by filtration methods using a pre-coated ilter at a temperature which approaches atmospheric temperature. However, the prior art methods for separating the insoluble material from the liquid coal extract have been for the most part unsatisfactory in the commercial sense.

It is also known by the prior art that the solvent extraction method is basically non-selective. In other words, the solvent which is utilized to convert the coal into liquid coal extract generally accomplishes substantially total solution of the solid coal Without regard to any selectivity as to more valuable or less valuable components coritained therein. Thus, when solid coal is dissolved with the conventional high boiling aromatic type solvent, it is technically very difficult to Separate the resulting liquid coal extract from the insoluble materials contained therein both organic and inorganic. Basically, these aromatic type solvents dissolve the coal indiscriminately thereby producing a liquid coal extract containing components with a low hydrogen content as Well as components with a high hydrogen content.

Therefore, since it is clear to those skilled in the art that the vast mineral reserves of bituminous coal represent an extremely important supply of energy, it would be desirable to improve upon the prior art processes, particularly the solvent extraction process, in order to render it more selective for the more desired valuable components and to reduce the cost of obtaining typical petroleum-type products from coal.

SUMMARY OF THE INVENTION Accordingly, it is an object of this invention to provide a method for the liquefaction of coal whereby valuable hydrocarbon products may be obtained therefrom.

It is a specific object of this invention to provide an improved method for subjecting pulverized coal to solvent extraction utilizing a novel two-stage solvent extraction technique.

It is another specific object of this invention to provide an improved method for liquefying coal via solvent extraction whereby more valuable hydrogen-rich components are obtained as a separate product stream from the hydrogen-lean components contained in the basic liquid coal extract in a facile and economical manner.

This invention basically comprises a method aspect, although system aspects will be noticeable to those skilled in the art. In accordance with the practice of one embodiment of the method aspect, there is provided a method for liquefying solid particulate coal which comprises the steps of: (a) contacting said coal with a selective first solvent under conditions suicient to convert said coal into liquid coal extract containing hydrogen-rich components and hydrogen-lean components; (b) separating the liquid coal extract from undissolved coal and from at least a portion of said rst solvent; (c) contacting said separated liquid coal extract from step (b) under extraction conditions with a second solvent selective for Said hydrogen-rich components; and, (d) recovering said hydrogen-rich components and said hydrogen-lean components as separate product streams.

Another embodiment of this invention includes the method hereinabove wherein said first solvent comprises aromatic hydrocarbons and said second solvent comprises a ketone.

A still further embodiment of this invention includes the method hereinabove wherein the extraction conditions of step (c) include the presence of a hydrogen-containing gas.

In other Words, the method of the present invention utilizes a novel two-stage solvent extraction technique wherein in the iirst stage crushed coal is contacted with an aromatic hydrocarbon solvent to produce a liquid coal extract dissolved in the solvent. The extract is then separated, at least in part, from the solvent and subsequently contacted with a dilferent type selective solvent in order to separate the hydrogen-rich components from the hydrogen-lean components. These separated components are then recovered as separate product streams.

DETAILED DESCRIPTION OF THE INVENTION The coal preferred for use in the practice of the present inventive method is of the bituminous type, such as Pittsburgh seam coal. More preferably, however, the

bituminous coalis a high volatile content coal having a volatile content greater than about by weight of the maf. coal (moisture and ash-free coal).

Suitable solvents for use in the first stage extraction step of the present invention are those which, preferably, are of the hydrogen-donor type and which are at least partially hydrogenated, such as the naphthenic-aromatic hydrocarbons. Preferably, the rst stage solvent is one which is in liquid phase at the recommended temperature and pressure for the extraction and/or pulverization step. Mixtures of hydrocarbons are generally employed and, preferably, are derived at least in part from the intermediate or final products obtained from subsequent processing following the practice of this invention.

Typically, these first stage solvent hydrocarbons or mixtures of hydrocarbons boil between about 260 C. and 425 C. Examples of suitable rst stage solvents are tetrahydronaphthalene (Tetralin), Decalin, biphenyl, methylnaphthalene, dimethylnaphthalene, mixtures of phenanthrene and anthracene, etc. Other types of solvents which may be added to the preferred rst stage solvents of this invention for special reasons include phenolic compounds, such as phenols, cresols, and xylenols. It is also to be recognized that in some cases, it may be desirable during a subsequent separation step, typically, prior to the sec ond stage extraction operation, to add an anti-solvent, Such as saturated parainic hydrocarbons, such as nhexane, to aid in the precipitation of tarry and solid residue from the liquid coal extract of the invention.

However, in the search of a suitable solvent for the rst stage liquefaction zone, it must be recognized that this solvent should have the ability to transfer hydrogen to the pulverized coal during this liquefying step. In other Words, it is a requirement of this invention that in the absence of added hydrogen-containing gas that the rich solvent leaving the first stage extraction zone having liquid coal extract dissolved therein, have a reduced hydrogen content compared to the hydrogen content of the lean solvent which is added to the first stage extraction zone, usually in admixture with the feed coal.

A particularly preferred embodiment of this invention includes the use of a selective solvent of the type described above for use in the first stage extraction zone during the pulverization step whereby relatively coarse size coal is reduced to granular coal of optimum size for extraction. This preferred embodiment is predicated on the theory that having the presence of a hydrogen-rich solvent during the pulverization step of the coal results in a substantial increase in the efficiency of the operation and in many cases results in a decreased use of the first stage solvent for obtaining the same quality and quantity of liquid coal extract.

With respect to the benefit gained from having the solvent present during the pulverization step, it is believed that at the point of shear for the crushing and grinding of the coal, the shear site is extremely reactive and hydrogen, therefore, can be transferred into that site more easily than if the coal is pulverized prior to contact with the solvent. In addition, the small particles of coal which are sheared away from a large lump immediately expose not only the highly reactive shear site to the solvent, but also exposes a relatively large surface area to the solvent, thereby enabling the small particles of coal to almost immediately dissolve in the solvent and become a part of the liquid coal extract. While not desiring to be limited by this theory, the practice of this preferred embodiment of the invention is at least explained to those skilled in the art so that further work may be used along these lines to further improve upon the inventive concepts contained herein.

Apparatus for use in pulverizing the lump or coarse coal feed as practiced in the present invention may be of any type known to those skilled in the art. Conventional ball mills or rod mills may be used with satisfactory results. Preferably, the apparatus must be able to pulverize lump or coarse coal in the presence of significant quantities of liquid solvent without diiculty. Those skilled in the art are familiar with the kinds of apparatus for processing wet solids and the crushing and grinding thereof, such that no detailed discussion of the apparatus need be presented herein. The primary requirement for crushing and grinding of the lump coal is that coarse coal usually having an average particle diameter in excess of 0.08 inch and, typically, about 0.25 to 2.0 inches must be processed thereto and reduced in size to an average particle diameter which would be reduced to an average particle size for -14 Tyler screen size. As used herein the term Tyler Screen refers in all instances to the commercial Tyler standard screens. The correlation between Tyler screen mesh and average particle diameter is as follows:

Average diameter of Tyler screen mesh: particles Davg in.

The conditions during the pulverization step may be varied widely according to the desires of those skilled in the art and practicing this invention. The temperature, of course, may be varied over a relatively broad range, from essentially ambient temperature to a relatively high ternperature. It is distinctly preferred in the practice of this invention that the temperature of the coal and the solvent be mainained at a relatively high temperature, say, from 300 C. to 500 C. The pressure, in similar manner, may be varied over an extremely wide range from atmospheric pressure to, say, 10,000 p.s.i.g. with a preferred pressure being about 100 p.s.i.g. or typically, about 70 p.s.i.g.

The operation of the pulverization equipment is preferably performed so that the oversized material; that is, greater in size than the -8 Tyler screen size, be separated and returned to the apparatus for further pulverization. The utilization of the closed circuit technique is well known to those skilled in the art and is preferred in the practice of this invention. Unless otherwise stated, closed circuit operation of the pulverization equipment will be deemed inherent in the practice of this invention.

The amount of solvent which is used in the first stage extraction of the present invention generally will range from 0.2 to 10 pounds of solvent per pound of coal. Satisfactory results may be obtained in utilizing approximately equal amounts of solvent to coal on a weight basis. In the practice of the preferred embodiment of this invention, the conditions during the pulverization step should be chosen such that the coarse coal is reduced in size to at least a -8 Tyler screen size and the solvent has a chance to react and dissolve the coal to an extent such that the coal particles are at least partially dissolved in the solvent. As more fully developed hereinbelow, the conditions are chosen in the pulverization step such that from 10% to 40% of the maf. coal is dissolved in the solvent with at least an additional 50% by weight being dissolved during the subsequent digestion zone.

The extraction of coal by means of a solvent is basically a partial conversion of the coal since not only is the coal reacted with the hydrogen, which is preferably transferred from the solvent, but there is also a solution phenomenon -which actually dissolves the coal which has accepted the hydrogen into the solvent phase. Therefore, as used herein, the term liquid coal extract, liquefied coal fraction, liquefied coal, or other words of similar import, is intended to include the liquid product which is obtained from the various steps in the practice of this invention and, generally, will be described on the basis of being solvent-free, even though a portion of the final liquid coal extract following the second extraction step comprises hydrocarbons suitable for use as the selective solvent in the first extraction zone. The practice of the present invention in both extraction zones is performed under conditions Which increase the kinetics of the solvation reaction while maintaining the components therein in primarily liquid phase; although, in some cases it may be desirable to practice the first step of the extraction operation in the presence of a vaporized solvent using a gaseous extraction technique.

Suitable preferred solvents for use in the second stage extraction zone belong broadly to the class of compounds known as ketones, i.e. oxidative derivatives of secondary alcohols. Examples of ketones which may be used satisfactorily in the second extraction zone include acetone, methylethyl ketone, methylbutyl ketone, dibutyl ketone, etc. Other suitable solvents for use in the second stage extraction include monocyclic aromatic hydrocarbons (e.g. benzene, toluene, xylene, etc.) and naphthenic hydrocarbons (e.g. methylcyclohexane, etc.). It is to be noted by those skilled in the art that these classes of secondary extraction agents was found to have no effect on converting solid coal to a liquid coal extract. Therefore, it is a requirement of this invention that the first stage extraction zone utilize those solvents which are applicable to the conversion of solid coal particles to liquid form and that the second stage extraction zone be strictly limited to a solvent which serves to separate the hydrogen-rich components from the hydrogen-lean components contained in thte liquid coal extract produced in the first extraction step. In other words, it was found that the ketones, as well as the other above-mentioned secondary solvents are selective for the hydrogen-rich components to the substantial rejection of the hydrogen-lean components. The choice of selective solvent depends upon the extraction conditions desired in the second extration zone. The initial boil point of the solvent should be above the temperature selected for the second extraction zone. For practical purposes, the temperature selected for the second extraction zone should be at least 30 C. below the critical boiling point of, for example, the ketone in order to maintain the components in primarily liquid phase. As a rule, the temperature for the second extraction zone should not exceed 450 C.

The operating conditions for the second extraction zone utilizing the ketone solvent include a temperature from 50 C. to 450 C., preferably, from 100 C. to 300 C., a pressure from 100 p.s.i.g. to 1000 p.s.i.g., preferably from 350 p.s.i.g. to 700 p.s.i.g., a solvent to feed ratio from 0.5 to by weight, a residence time from 0.5 LHSV to 5.0 LHSV and, preferably, the presence of a hydrogencontaining gas in an amount from 500 s.c.f. to 5000 s.c.f. per barrel of liquid feed to the second extraction zone. These conditions should be sufficient to substantially separate, on a selective basis, the hydrogen-rich components from the hydrogen-lean components which are contained in the liquid feed to the second extraction zone.

The liquid coal extract obtained from the first extraction zone will contain compounds of widely varying physical characteristics since the first extraction step is relatively non-selective. However, the liquefied coal may be characterized as being composed of ybasically two fractions, to wit: a hydrogen-rich fraction and a hydrogenlean fraction. As used herein, the term hydrogen-lean components or words of similar import are intended to include those components which are basically insoluble in benzene. The hydrogen-lean components will typically have an average molecular weight of about 1500 and contain hydrogen on a weight percent basis from about 3% to about 5%. On the other hand, as used herein, the term hydrogen-rich components or words of similar import are intended to include those components which are basically soluble in benzene. The hydrogen-rich components,

therefore, will typically have a molecular weight of less than 1500, e.g. from 300 to 1000 and, therefore, a hydrogen content on a weight basis in excess of 5%, e.g. from 6% to 9% by weight. The above characteristics of the two major fractions contained in liquefied coal, of course, are influenced to some extent by the solvent extraction conditions used in the first stage extraction zone including the depth of extraction employed in this first step. It is recognized, however, that the hydrogen-lean component characteristic of the liquid coal extract will only be influenced slightly 1-by the extraction conditions, but will be influenced considerably by the ^kind of coal which is utilized as feed to the inventive process.

In summary, therefore, following the size reduction step wherein at least a portion of the coarse particulate coal has been dissolved in the first selective solvent and the oversized solid materials have been separated. The eflluent product comprising first selective solvent having dissolved therein liquid coal extract and containing undissolved solid coal is passed, preferably, into a digestion zone for the substantial conversion of the coal into liquefied coal.

The operating conditions for the digestion zone include a temperature from 300 C. to 500 C., a pressure from atmospheric to 10,000 p.s.i.g., a solvent-to-coal weight ratio from 0.2 to 10, and a residence time from 30 seconds to 5 hours, sufiicient to dissolve coal such that a total in excess of 50% by weight of maf. coal has been liquefied.

It is to be noted that the temperature and pressure conditions during the digestion step may be the same, may be higher, may be lower, or may be any different configuration desired :by those skilled in the art, of those conditions maintained in the pulverization zone. It has been satisfactory in the practice of this invention that the temperature and pressure in the digestion zone be maintained essentially at the same level as the temperature and pressure maintained in the pulverization zone.

Since the purpose of the digestion zone, including the preferred embodiment using the first selective solvent in the pulverization and digestion zones, is to substantially complete the conversion of the solid coal into a liquid coal extract, it may be desirable to add to the digestion zone additional solvent, add a hydrogen-containing gas to the digestion zone, and/or utilize a catalyst in the digestion zone. If a catalyst is required or desired, it may be of the conventional hydrogenation type either homogenous or heterogenous and may be introduced into the pulverization zone and/or digestion zone in admixture with the liquid first selective solvent or with the solid coal. Those skilled in the art from a knowledge of the characteristics of the coal, of the solvent, and of the properties desired for the end product will know from the teachings presented herein whether or not it would be desirable to use any or all of these features in the pulverization and/or digestion zone.

Examples of conventional hydrogenation catalyst which may be used in the first extraction zone include a cobaltmolybdate, a nickel molybdate, a nickel tungstate or any other hydrogenation catalyst which can operate in the presence of sulfur-containing charge stocks and is applicable to the solvent-coal system environment maintained in the first extraction zone, including the use of a slurry-catalyst system.

Hydrogenation in the digestion zone, generally, accomplishes the following functions: transfer of hydrogen directy to coal molecules; transfer of hydrogen to hydrogen-donor molecules to coal molecules; and combinations of the above. Homogenous catalyst may be introduced with the coal or with hydrogen-donor compounds in the pulverization step or in the digestion zone. By way of emphasis, as used herein, the term first extraction zone is intended to include the pulverization step, the digestion step or the combined pulverization-digestion step.

After separation of the undissolved coal residue of at least a portion of the first selective solvent and catalyst, if any, from the total effluent of the digestion zone, the liquid coal extract is now contacted in a second extraction zone under extraction conditions with the previously mentioned second selective solvent of the ketone class. A residue comprising hydrogen-lean components is removed from the second extraction zone for use as fuel or for the conversion thereof into relatively pure hydrogen gas by use of the water-gas reaction. The produced hydrogen then may, of course, be utilized within the inventive method circuit as described herein. The rich solvent from the second extraction zone comprising the ketone having dissolved therein the hydrogen-rich components is further processed by means known to those skilled in the art, such as fractionation, in order to separate the liquid coal extract into more valuable products, such as relatively light hydrocarbons, relatively heavy hydrocarbons, chemicals, fuel, etc., the utility of which is well known to those skilled in the art. As previously mentioned, a portion of the separated products from the liquid coal extract may be satisfactory for use as at least a portion of the first selective solvent utilized in the rst extraction zone as described herein.

The invention may be more fully understood with reference to the appended drawing which is a schematic representation of apparatus for practicing the detailed and preferred embodiment of the invention.

DESCRIPTION OF THE DRAWING Referring now to the drawing, coarse coal having an average particle diameter generally in excess of 0.08 inch is introduced into the system via line 10. A suitable selective first solvent enriched in hydrogen content, such as Tetralin, is introduced into admixture with the coarse coal from line 11, the major source of which is more fully discussed hereinafter. As previously mentioned, the over-sized solid material from the pulverization zone is also preferably returned to the pulverization zone via line 12. The entire admixture of coarse coal and first selective solvent is passed via line 13 into mill 14 which conventionally may be of the ball mill type adaptable for the use in the presence of a liquid according to means Well known to those skilled in the art.

Suitable pulverization conditions include a temperature of about 380 C., a pressure of about 70 p.s.i.g., and a solvent to coal ratio of about 1.0 is maintained in mill 14, such that the coarse coal is reduced to an average particle diameter between 0.08 and 0.04 inch, and at least a portion of the coal, say, about 17% by weight is dissolved into the Tetralin.

The eluent from mill 14 containing Tetralin having dissolved therein the liquid coal extract and containing undissolved coal of proper small particle size and undissolved coal of oversize is passed via line 15 into separator 16 which may be of the cyclone type. Conditions are maintained in separator 16 whereby the oversized coal particles, preferably, in admixture with at least a portion of the liquid material is removed via line 12 and returned to rnill 14 in a manner previously discussed.

AFirst selective solvent, e.g. Tetralin, having dissolved therein at least a portion of the coal plus undissolved pulverized coal is admixed, preferably, with a hydrogencontaining gas from line 26 and passed via line 17 into digestion zone 19 which may be a jacketed stirred type vessel. Added Tetralin solvent, if any, may be introduced into the system via line 18 in an amount suicient to maintain the solvent to coal ratio at the desired level and/or to maintain the hydrogen content present in digester 19 at a suiiiciently high level. Furthermore, hydrogenation catalyst (from means not shown) advantageously may be used in the digestion step. Make-up hydrogen, if any, may be added to the system via line 27. Preferably, the amount of hydrogen present in the digestion zone is from 1,000 to 100,000 standard cubic 8 feet per barrel of coal-solvent mixture entering digester 19 via line 17.

The entire effluent from digestion zone 19 is passed via line 20 into filtration zone 21 wherein solid residue, including solid hydrogenation catalyst, if any, is separated from the liquid material and removed lfrom the system vi-a line 22. Filtration zone 21, preferably, is a rotary filter device, pre-coated with conventional lter aids and is, typically, operated at a pressure from 50 to p.s.i.g. suficient to effect removal of substantially all of the suspended solid matter including undissolved carbonaceous matter. The tilter cake is washed and dried by conventional means to recover absorbed liquid material. The mother liquor, including any gaseous materials, is removed from filtration zone 21 via line 23. The material in line 23 comprises solvent having dissolved therein liquid coal extract comprising both hydrogen-rich components and hydrogen-lean components.

As previously mentioned, if desired, by means not shown, an anti-solvent, such as a light hydrocarbon of the hexane type, may be added to filtration zone 21 in an effort to further aid in removing tars and solid materials from the desired solvent and liquid hydrocarbons making up the liquid coal extract. If an anti-solvent is used, the material in line 23 will also contain such added light hydrocarbons.

The material from ltration zone 21 is passed through line 23 into separator 24 which may comprise one or more separation vessels, including fractionation columns for the separation of the hydrogen gas, added light hydrocarbons, if any, from the other hydrocarbonaceous material. Although separation zone 24 has been shown in a position following filtration zone 21, it is within the concepts of the present invention to arrange for the separation of Igaseous materials, such as hydrogen gas, prior to filtration zone 21. In other words, another embodiment of this invention would include a gaseous (eg. hydrogen) separation zone, not shown, in line 20 between digester 19 and lter 21. However, for purposes of this illustrative embodiment only, separation zone 24 has been placed between filtration zone 21 and secondary extraction zone 29, more fully discussed hereinbelow.

In keeping with the present illustrative embodiment, hydrogen gas is removed from separation zone 24 via line 26 and returned to digester 19. Preferably, that portion of the normally liquid material entering separation zone 24 from line 23 which comprises first selective solvent component (Tetralin) is Withdrawn rvia line 28 and returned to the digestion zone and/o1 pulverization zone, as previously discussed. Therefore, the liquid coal extract, substantially free of solid matter, free of hydrogen gas, and free of normally gaseous hydrocarbons and, preferably, free of first selective solvent components, is removed from separator 24 via line 2S.

The material in line 25, comprising liquid coal extract substantially free of solvent (although, it is recognized that a portion of the liquid coal extract may be suitable for use as the first selective solvent more fully developed hereinbelow) is passed into secondary extraction zone 29. If desired, a portion of the material in line 25 may be returned via line 41 into line 28 for recycle to the digestion zone 19 and/or pulverization zone 14.

Extraction zone 29 is maintained under extraction conditions, including the presence of a ketone class solvent which enters the system via line 39 and a temperature of about C., a solvent to extract ratio of about 3, a pressure of about 500 p.s.i.g., and, preferably, having introduced into the zone added hydrogen gas via line 40 in an amount of about 100 scp/barrel of extract enterlng zone 29. A residue comprising hydrogen-lean components is withdrawn from second extraction zone 29 via line 30 and disposed of as fuel or as a source of additional hydrogen through a water-gas reaction (by means not shown).

The rich solvent comprising ketone having dissolved therein hydrogen-rich components, such as naphthenic and naphthenic aromatic hydrocarbons and gasoline boiling range hydrocarbons is passed via line 31 into separation zone 32 which comprises a plurality of separatory means including fractionation colurnns` Conditions are maintained in separation zone 33, which are known to those skilled in the art from the teachings herein, sufiicient to recover the ketone solvent which is withdrawn via linc 36, to recover the normally gaseous materials, if any, which are withdrawn from line 35, to recover and separate naphthenic and naphthenic karomatic type hydrogen-rich components suitable for use as the first selective solvent which is withdrawn via line 37 and returned to digestor 19 and mill 14 in the manner previously discussed, with the net production of light hydrocarbon components being withdrawn from the system via line 38, and to separate and recover the relatively heavy hydrocarbons, also hydrogen-rich, which are withdrawn from the system via line 34.

The residual fraction in line 34 comprises basicallyl a heavy fuel oil and is used desirably in a fuel system. Added first selective solvent, such as Tetralin, external of the process may be added to the inventive system via line 32, if desired.

The invention may be now fully understood by reference to the following examples which illustrate the dual solvent technique:

lEXAMPLE l Wt. percent hydrogen lst stage, liquid coal extract 5.2 2nd stage, final extract 5.7

It is evident from this data that the second solvent extraction step significantly upgraded the liquefied coal, e.g., the 0.5% increase in hydrogen content.

Similar results would be expected from using any of the other above defined brand class of ketone secondary* solvents.

EXAMPLE 2 Bituminous particulate coal was pulverized and divided into two batches. The first batch was liquefied by contacting the coal with Tetralin in the presence of hydrogen gas and a hydrogenation catalyst. Operating conditions and catalyst were as defined hereinabove. A first liquid coal extract was obtained. The second batch was liquefied under similar conditions except that no hydrogenation catalyst was present. A second liquid coal extract was also obtained. Analytical data was as follows:

Batch No.

Liquefied Liquetied with without catalyst catalyst Percent extract (m a i basis) S9. 85. 5 Percent C, wt.-. S7. 70 86.00 Percent H, Wt... 7. 42 7. 18 Molecular Weight- 481 503 Each extract was filtered to remove solid residue and ash and then subjected to a second extraction step using cyclohexene at C. for two (2) hours. The secondary solvent Awas removed by distillation and each extract and following results:

SECOND EXTRACTION Original Extract No 1 2 l. Percent cyclohexene soluble. 40 40 Percent C, Wt 86. 36 85, 4G Percent H, Wt. 8.33 8. 2M Mol. wt 340 348 2. Percent benzene soluble. 88 36 Percent C, wt 84. 32 84. 97 Percent H` wt.. 8. 45 7. 20 Mol. wt 605 553 3. Percent insoluble residue- 32 34 Percent C. Wt 85. 55 84. 41

` Percent, H w1: 5. 52 5. 87 Percent ash- Nil Nil n Mol. wt 1, 1,130 1 and 2 percent combined sol. prod. 78 76 Percent H. wt S. 30 7. 75 Mol. wt 472 450 Percent soluble based on original m.a.f. basis 70 65 Percent soluble based on original coal charge. 66 61 The above data shows the benefits obtained by using the dual solvent technique. First, each secondary extract (a final extract) had a significantly higher hydrogen content; e.g. Batch 1, from 7.42% to 8.39%. Second, the insoluble residue in both instances 'was substantially ash-free, and since its properties were similar to the original coal, it could be recycled essentially to extinction. Third, the effect of the catalyst when Batch l is compared to Batch 2, is clearly evident. Batch l extract had a higher hydrogen content (c g. 8.39 vs. 7.75) and a higher overall yield of liquefied product (c g. 70% vs. 65%).

Similar results would be expected by using a single ring aromatic hydrocarbon such as benzene as the secondary solvent.

PREFERRED EMBODIMENT From theV discussion presented hereinabove, the preferred embodiment of the present invention includes a method for liquefying coal which comprises the steps of: (a) admixing coarse size bituminous coal with a first solvent selective for dissolving coal; (b) introducing the solvent-coarse coal admixture into a pulverization zone under-,conditions including a temperature from 300 C. to 500 C., a pressure from atmospheric to 10,000 p.s.i.g., solvent to coal ratio from 0.2 to 10.0 sufiicient to reduce said coarse coal to at least a 8 Tyler screen size and to partially dissolve coal into said solvent; (c) passing the pulverized coal-solvent product including dissolved coal into a liquefaction zone maintained under conditions sufficient to dissolve coal such that a total in excess of 50% by weight of m.a.f. coal is liquefied as liquid coal extract; (d) separating said liquid coal extract from undissolved coal and from said solvent thereby producing a substantially solvent-free liquid coal extract; (e) introducing said solvent-free liquid coal extract into an extraction zone maintained under extraction conditions including a second solvent selective for hydrogen-rich components in said extract; (f) withdrawing from said extraction zone a rich solvent having hydrogen-rich liquid coal components dissolved therein; and, (g) recovering said hydrogen-rich components in high concentration.

A distinctly preferred embodiment includes the method hereinabove wherein said first solvent comprises naphthalenic hydrocarbons and said second solvent comprises a ketone.

-It will be recalled that hydrogen gas was preferably used during the digestion step and during the secondary extraction step. It is important to note that these are separate and distinct hydrogenation operations. In order to carefully control the degree of hydrogenation, it is convenient to use the I-factor analysis for determining the degree to which hydrogen has been added to the materials to be hydrogenated in both digestion zone 19 and extraction zone 29. This analytical technique permits the characterization of various types of aromatics in a hydrocarbon mixture by means of the I-factor analysis. The technique utilizes mass spectrometer analysis emto the molecular weight of the aromatic compound. Thus,

the technique permits characterization of the aromatic hydrocarbons by means of the general formula CH3 J where J is the herein referred to Xl-factor for the practice of the present invention. The following table shows the relationship between the J-factor and the type of aromatic.

J-factor number: Type of aromatic hydrocarbon 6 Alkyl benzenes and benzene.

8 lndanes, Tetralin.

l0 lndenes.

l2 Alkyl naphthalenes and naphthalene. 14 Acenaphthenes, tetrahydroanthracene. 16 Acenaphthalenes, dihydroanthracenes. 18 Anthracenes, phenanthrenes.

Using this J-factor analysis in characterizing the hydrotreating step of the present invention allows for the optimum treatment of said solvent to produce a high quality hydrogen enriched solvent for use in converting coal into liquid coal extract. However, as previously mentioned, the important control technique of the present invention is that in the absence of added hydrogen gas the hydrogen content of the initial solvent charged to the first extraction zone in admixture with the coal is greater than the solvent leaving the pulverization zone having dissolved therein the liquid coal extract. ln similar man-A ner, the other control technique is that the hydrogen content of the solvent which has been recovered from the efliuent of the pulverization zone is less than the hydrogen content of the solvent leaving the digestion zone land being recycled to the pulverization zone. The I-factor analysis is simply a convenient means for optimizing the hydrotreating steps in the practice of the invention.

The invention claimed:

l. Method for liquefying solid particulate coal which comprises the steps of:

(a) contacting said coal with a first solvent selective for dissolving coal under conditions sufficient to convert said coal into liquid coal extract containing hydrogen-rich components and hydrogen-lean components;

(b) separating the liquid coal extract from undissolved coal, and from at least a portion of said lirst solvent;

(c) contacting said separated liquid coal extract from step (b) under extraction conditions with a second solvent different than said first solvent and selective for said hydrogen-rich components and being selected from the group consisting of ketones, mono- 12 cyclic aromatic hydrocarbons and naphthenic hydrocarbons; and

(d) recovering said hydrogen-rich components and said hydrogen-lean components as separate product streams.

2. Method according to claim 1 wherein said first solvent comprises polycyclic aromatic hydrocarbons or polycyclic naphthenic-aromatic hydrocarbons.

3. Method for liquefying coal which comprises the steps of (a) admixing coarse size bituminous coal with a first solvent selective for dissolving coal;

(b) introducing the solvent-coarse coal admixture into a pulvcrization zone under conditions including a temperature from 300 C. to 500 C., pressure from atmospheric to 10,000 p.s.i.g.. solvent to coal ratio from 0.2 to 10.0 sufficient to reduce said coarse coal to at least a -8 Tyler screen size, and to partially dissolve coal into said solvent;

(c) passing the pulverized coal-solvent product including dissolved coal into a liquet'action zone maintained under conditions sutiicient to dissolve coal such that a total in excess of 50% by weight of m.a.f. coal is liquefied as liquid coal extract;

(d) separating said liquid coal extract from undissolved coal, and from said solvent thereby producing a substantially solvent-free liquid coal extract;

(e) introducing said solvent-free extract into an extraction zone maintained under extraction eonditions including the presence of a second solvent different than said lirst solvent and selective for hydrogen-rich components in said extract and being selected from the group consisting of ketones, monocyclic aromatic hydrocarbons and naphthenic hydrocarbons;

(f) withdrawing from said extraction zone a rich solvent having hydrogen-rich liquefied coal components dissolved therein; and,

(g) recovering said hydrogen-rich components in high concentration.

4. Method according to claim 3, wherein said first solvent comprises naphthalenic hydrocarbons and said second solvent comprises a ketone.

5. Method according to claim 2 Ywherein said conditions of step (c) includes the presence of a hydrogen-contain ing gas.

References Cited UNlTED STATES PATENTS 3,162,594 12/ 1964 Gorin 208-8 1,822,349 9/ 1931 Jannek 208-8 2.476.999 9/ 1949 Orchin 208-8 3,018,241 1/ 1962 Gorin .V-.. 208-8 3,018,242 l/ 1962 Gorin 208-8 DELBERT E. GANTZ, Primary Examiner V. O'KEEFE, Assistant Examiner U.S. Cl. X.R. 208-337 

